Fuel cell including porous palladiumsilver alloy electrode



March 5, 1968 .1. FORTEN FUEL CELL INCLUDING POROUS PALLADIUM-SILVERALLOY ELECTRODE Filed Aug. 19, 1965 uw zm N 2202 INVENTOR JEraE/mForers/v United States Patent 3,372,061 FUEL CELL INCLUDENG POROUSPALLADIUM- SILVER ALLOY ELECTRODE Jeremy For-ten, New York, N.Y.,assignor to Leesona Corporation, Warwick, Rl, a corporation ofMassachusetts Continuation-impart of application Ser. No. 143,561, Oct.9, 1961. This application Aug. 19, 1965, Ser. No. 480,886

5 Claims. (Cl. 13636) This application is a continuation-in-part of mycopending application Ser. No. 143,561 filed Oct. 9, 1961, nowabandoned.

This invention relates to improved electrodes for use in anelectrochemical device and, more particularly, to reduction electrodesfor use in a fuel cell. The electrodes of the invention comprisepalladium alloys containing from about 0.25 to 90 atomic percent silver.

In the prior art, palladium has been recognized to have excellentproperties as an activator for electrodes in an electrochemical device,and, more particularly, in fuel cell electrodes. Palladium, however, isan expensive material detracting from its use in a commercial cell.Therefore, an effort has been made in the art to obtain electrodesproviding equivalent performance, or substantially equivalentperformance to palladium, but which are cheaper in construction.Additionally, an effort has been made to obtain materials havingimproved electrochemical activity, under given conditions, using theelectrochemical activity of palladium as a reference point. it has nowbeen found that porous palladium-silver alloy electrodes containing fromabout 0.25 to 90 atomic percent silver are superior in performance topalladium electrodes, or provide substantially equivalent performancebut being considerably more economical due to the lower cost of silver,are commercially feasible.

Accordingly, it is an object of the present invention to provide fuelcell reduction electrodes which have good activation characteristics.

It is another object of this invention to provide reduction electrodesfrom a relatively inexpensive palladium alloy containing from about 0.25to 90 atomic percent silver.

These and other objects of the invention will become more readilyapparent from the following detailed description, with particularemphasis being placed on the illustrative examples.

According to the present invention, it has been found that alloys ofpalladium and silver have substantially different properties in theactivation of a fuel cell reduction electrode over pure palladium orpure silver. Thus, it has been found that palladium-silver alloyscontaining from as low as 0.25 atomic percent silver and as high as 76atomic percent silver provide greater electrochemical activity than purepalladium, although silver is much less effective than palladium (by afactor of more than 65) as an activator in the fuel cell reductionelectrode. Moreover, electrodes having up to 90 atomic percent silverand only atomic percent palladium provide electrochemicalcharacteristics approaching that of pure palladium. Such electrodes areattractive commercially in view of their relatively lower cost. Althoughpalladium alloy electrodes containing from about 0.25 to 90 atomicpercent silver can be employed according to the instant invention,alloys containing from about 0.25 to 35 atomic percent silver arepreferred with op- 3,372,001 Patented Mar. 5, 1968 timum ratios beingobtained from electrodes containing 1.0 atomic percent silver and atomicpercent silver.

To demonstrate the performance of the described alloys as oxygenelectrodes, tests were performed at C. in one normal potassium hydroxidesaturated with oxygen. Current was recorded at a constant voltage of0.85 volt versus a hydrogen electrode in the same system. The test whichused highly polished fiat sheets are only for establishing the relativeelectrochemical activity as a function of the alloy composition.Understandably, higher current densities are obtained when the alloysare formed into porous sintered structures. The following data for thestated alloy were obtained:

Pd-Ag alloy, Current density,

percent Ag an/cm? 0.0 340 The data which is also plotted as FIGURE 1 ofthe drawing demonstrates the unusual behavior of the palladium-silveralloys. Thus, from the plot, it is seen that as little as 0.25 atomicpercent silver increases the current density from 340 microamps persquare centimeter for pure palladium to 560 microamps. When one atomicpercent silver is used, a peak performance of 660 microamps per squarecentimeter is obtained. At two atomic percent silver, a reducedperformance of 500 microamps per square centimeter is obtained; however,the electrode is still superior to pure palladium. A second peak and anoptimum performance of 690 ,ua/cm. is obtained at about 35 atomicpercent silver and atomic percent pall-adium. Although the performancewith increased amounts of silver above 35 atomic percent decreases, theperformance is still unexpectedly better than pure palladium atpercentages up to 62 atomic percent silver, as apparent from the drawingand due to the lower cost of the alloy, has a decided commercialadvantage. The drop in performance is continuous until, at approximately76 atomic percent silver, the performance of the alloy is almostidentical to pure palladium. The drop in performance thereaftercontinues, with pure silver showing a current density of only 5microamps per square centimeter. Even at atomic percent silver, theperformance of the electrode is increased by a factor of about 40' overpure silver. Although this performance at 90 atomic percent silver isbelow that of pure palladium, in view of the reduced cost of theelectrode, its use is commercially attractive. The data plotted inFIGURE 1, which is relative data, is highly unexpected and would not bepredicted from the published fuel cell or electrochemical literature.There is no reason to expect that an alloy of palladium and silver wouldbe better than pure palladium, or that as little as percent palladiumwould increase the performance characteristics of a silver electrode bya factor of 40.

The electrodes of the invention, which comprise Pd-Ag aloys with from0.25 to 90 atomic percent silver, can be substantially non-porousstructures whereby the oxidizing gas is caused to flow against andaround the electrode, or the structure can be porous and the oxiclizinggas passed through the electrode. The recovery from polarizationconditions of such electrodes to open circuit potential is rapid and theelectrodes are resistant to corrosion and poisoning by impurities in theoxidant gas stream and in the electrolyte.

The paladium-silver alloys of the present invention can be employed inthe preparation of porous electrodes by methods known in the art. Thus,alloy powders having a particle size of from about 1-100 microns can becompacted and sintered to form a porous structure. More specifically, aporous electrode can be prepared by placing a ring with a lip of theproper thickness, usually from about 0.003 inch to about 0.30 inch, overan alumina coated disc, pressing the powder of the selected particlesize on the disc and striking off the excess powder so that a layer ofpowder approximately the thickness of the lip remains. The powders arethen compacted, as for example, by vibratory techniques, and sintered.In the compaction operation, the pressure at which the alloy powders arepressed can vary over a relatively wide range. Thus, compaction can becarried out at pressures as low as about 500 psi. and as high as about8,000 p.s.i.

The sintering of the electrodes after compaction can be carried out inany of the usual sintering furnaces, as for example, a retort inclinedfurnace. It is possible and often desirable to sinter the samplesaccording to a predetermined cycle, thus, the cycle may consist ofpreheating to a temperature of 200 F. for a short time and thereaftersintering at a temperature of from about 1,0002,400 F. for a relativelylonger period of time before cooling. The sintering can be carried outfor varying lengths of time depending upon the sintering temperature.The sintering operation, often can be performed with advantage in anatmosphere of hydrogen or forming gas. At times, it may be preferable tocarry out the sintering operation in air or a vaccum.

The reduction electrodes of the present invention are usually employedin cells operating at temperatures of from about 160 C., however, thetemperatures to a large extent depend upon the fuel employed in the fuelcell as well as the activating material used at the fuel electrode. Ithas been found that the fuel cell systems employing the reductionelectrodes of the instant invention can be operated within thetemperature range of from about 20650 C. with satisfactory performance.

The electrodes, since they are relatively inert to chemical reaction,can be employed in fuel cells using known prior art electrolytes such asthe aqueous alkaline hyroxides and carbonates. Virtually, any ionicconductor Which remains substantially invariant under the operatingconditions of the cell can be employed. Additionally, the electrodes canbe operated with known oxidizing gases including pure oxygen or air.

To more particularly describe the preparation of the claimed electrodesand to demonstrate their utility in fuel cells, Examples 1-3 inclusiveare set forth as preferred embodiments of manufacturing a porouselectrode;

and Examples 4-6 demonstrate the use of the prepared electrodes in afuel cell system.

EXAMPLE 1 Palladium alloy powders having a particle size of from about20-35 microns in diameter and containing 35 atomic percent silver aresifted into a ring having a 0.5 inch lip and compressed at a pressure of3,000 psi. for eight minutes. Thereafter, the structure is sintered at atemperature of 1400 F. for a period of 40 minutes. The resultantstructure is a substantially homo-porous disc.

EXAMPLE 3 A powdered palladium alloy containing atomic percent silverand having a particle size of from about 5-10 microns is sifted into aring having a lip of from about 0.3 inch. The disc is compacted at apressure of 2,000 psi. and thereafter sintered for minutes at 1200 C. inan inclined retort furnace. The resultant disc is substantiallyhomo-porous.

EXAMPLE 4 A fuel cell was devised in a suitable housing employing theelectrode of Example 1 as the reduction electrode, a non-porous 25Ag-74% Pd-1% Nb hydrogen diffusion anode and using an eight molaraqueous potassium hydroxide electrolyte. The cell when operated atatmospheric pressure and a temperature of 55 C. employing hydrogen asthe fuel and air as the oxidant demonstrated excellent polarizationcharacteristics.

EXAMPLE 5 A fuel cell system having the reduction electrode of Example2, a non-porous 25% silver-75% palladium hydrogen diffusion anode andusing a 28% aqueous potassium hydroxide electrolyte is devised in asuitable housing. The cell when operated at one atmosphere and atemperature of C. using hydrogen as the fuel and air as the oxidantdemonstrated good electrochemical performance characteristics.

EXAMPLE 6 A fuel cell is constructed in a suitable housing having acathode prepared as in Example 3, a non-porous 25% silver-75% palladiumhydrogen diffusion anode and using a five molar aqueous potassiumhydroxide electrolyte. The cell when operated at atmospheric pressureand at a temperature of 95 C. using hydrogen as the fuel and air as theoxidant demonstrated good electrochemical performance characteristics.

The illustrative examples are given as preferred embodiments and theinvention is nOt to be construed as being limited .thereby. It ispossible to produce still other embodiments of the invention withoutdeparting from the spirit of the invention and scope of the appendedclaims.

What is claimed is:

1. A fuel cell for the direct generation of electricity from a fuel andoxidant comprising a housing, a porous cathode comprising apalladium-silver alloy consisting essentially of palladium and fromabout 0.25-62 atomic percent silver, an anode and an electrolyte betweensaid cathode and anode.

2. The fuel cell of claim 1 wherein the palladiumsilver alloy consistsessentially of palladium and from References Cited about 0.25-35 atomicpercent silver. UNITED TATES PATENT I 3. The fuel cell of claim 1wherein the palladium- S S silver alloy consists essentially ofpalladium and about 3180762 4/1965 Oswm 136'86 L0 atomic percent silver5 3,062,909 11/ 1962 Reutscm 13686 4. The fuel cell of'claim 1 whereinthe palladium- 3,092,517 6/1963 Oswm 136 86 silver alloy consistsessentially of palladium and about about 0.251() atomic percent silver.ALLEN CURTIS, primary Examineh 5. The fuel cell of claim 1 wherein thepalladium- I silver alloy consists essentially of palladium and about 3510 WINSTON DOUGLAS Examme' atomic percent silver.

1. A FUEL CELL FOR THE DIRECT GENERATION OF ELECTRICITY FROM A FUEL ANDOXIDANT COMPRISING A HOUSING, A POROUS CATHODE COMPRISING APALLADIUM-SILVER ALLOY CONSISTING ESSENTIALLY OF PALLADIUM AND FROMABOUT 0.25-62 ATOMIC PERCENT SILVER, AN ANODE AND AN ELECTROLYTE BETWEENSAID CATHODE AND ANODE.